1. Field of the Invention
This invention relates generally to batteries and particularly to lead-acid batteries and manufacture thereof. In such batteries liquid acidic electrolyte must be sealed inside the battery case to prevent leakage of the electrolyte from the battery case around a battery post or terminal extending through the case and via which electrical energy is withdrawn from the battery. While the primary thrust of the invention is towards the battery art, particularly the lead-acid battery art, the invention has applicability wherever an electrically conductive, generally metal member must be provided with a liquid-tight seal where the metal member passes through a wall, particularly a wall of an electrically insulative case, containing liquid of high or low pH or an environment otherwise corrosive to conventional sealing materials and techniques.
2. Description of the Prior Art
Mechanical fluid sealing, in the electro-chemical, corrosive environment within a battery, at lead-acid battery terminals or posts, particularly at the positive terminal post or electrode, has presented a serious problem to the battery industry for many years. Post failure structural and/or leakage at the post seal cannot be tolerated.
Over thirty years ago G. W. Vinal, in noting the problem, summarized the seals and techniques which were then used in Storage Batteries, published by John Wiley & Sons, Inc., New York, N.Y., copyright 1955. As Vinal observed, "the method of sealing the terminal posts at the point where they pass through the cover is a matter of great importance. Unless the posts are satisfactorily sealed, they are likely to work loose in the cover and cause leakage of the electrolyte."
At that time annular sleeves or flanges were typically used about the battery post to provide a seal between the post and the cover or case. If the case was hard rubber, the sleeve or flange could be force-fitted into place, but only with exercise of great care to avoid damage to the soft lead metal post. Another approach was to fit the post with an externally threaded annular sleeve which threadedly engaged a tapped hole in the hard rubber battery case. Yet another approach was to provide threads on the lead post itself and a tapped hole in the battery case or in an insert fitted into a bore through the battery case. Still another approach was to provide threads on the post with a nut thread-engaging the post and pressing down tightly on the battery case top as the nut was tightened. Sometimes the threaded battery post was further provided with a flange around the post portion inside the battery case; tightly threading the nut on to the post pulled the post flange upwards, into tight engagement with the underside of the case top. A soft gasket could be provided between the flange and the case top as additional structure to protect against electrolyte leakage at the post.
As an alternative to the various thread arrangements, a metal ring, selected from an alloy which was harder than the load post, was burned about the exterior of the post, sealed with grease and urged against a soft gasket positioned between the metal ring and the battery cover exterior. The portion of the post inside the case was then deformed to retain the post in position with the ring squeezing the gasket against the case.
While these constructions have proved less than satisfactory, some of them remain in use even today, for want of a better post seal.
A more recent approach generally disclosed in the Bell System Technical Journal, Volume 49, No. 7, page 1405, copyright 1970 and in U.S. Pat. No. 3,652,340 has utilized a "relatively rigid adhesive" epoxy polymer resin coating applied to a clean load post over a large post area (column 2, line 65). The adhesive epoxy polymer coating is surrounded with a shrink-fitted, flexible, inflatable rubber tube which may also be further secured to the adhesive epoxy polymer resin coating by a butyl adhesive (column 3, line 29). The lower portion of the rubber tube forms a bag secured using the butyl adhesive to an annular flange of the battery case surrounding and extending inwardly from the battery case post orifice (column 3, line 38). There may also be provided a rib or dam inside the rubber tube so that the adhesive epoxy polymer resin coating may be applied to the post with the dam serving to limit downward flow of epoxy polymer resin along the post (column 4, line 5). When the rib or dam is used, the tube is not shrink-fitted to the post but is merely positioned about the post prior to pouring the epoxy, to leave a void for the epoxy to occupy; in this case the flexible, inflatable tube acts as a mold for the adhesive epoxy polymer resin coating (column 4, line 4). In either case, the flexible inflatable tube is physically separated from the battery post along the axial length of the flexible, inflatable tube by the adhesive epoxy polymer resin coating; the seal intended to prevent escape of the electrolyte is provided by an adhesive coating between the flexible, inflatable tube and both the post and the battery case. Alternatively, depending upon the design life of the cell, a suitable oxidation resistant grease may be used for the adhesive coating instead of the epoxy resin (column 2, line 74). The parenthetical reference numerals in this paragraph refer to columns and lines in the U.S. Pat. No. 3,652,340.
While the U.S. Pat. No. 4,652,340 approach has had some acceptance, the approach has not proven sufficiently reliable, particularly in small batteries where the area of adhesive rigid epoxy polymer resin coating-post contact is necessarily relatively small. The poor reliability of such epoxy seals has apparently been due, at least in part, to poor bonding at the rigid adhesive epoxy polymer resin coating post interface, between the rigid adhesive epoxy polymer resin coating and the lead or lead alloy of which the post is fabricated.
A variation on the old threaded post approach disclosed by vinal is presented in U.S. Pat. No. 4,245,014 in which the post itself is not initially threaded; rather a self-threading or self-tapping, sealant-containing member or nut is forceably threaded onto the post, cutting threads into the lead or lead alloy post and releasing sealant while engaging the post. This is combined with a case cover having a depression formed about the post orifice, which depression is filled with epoxy after the sealant-containing member is threaded into engagement with the post. The post is also equipped with a shoulder for abutting the underside of the case cover depression; a rubber O-ring is positioned around the post, squeezed between the post shoulder and the underside of the case cover depression.
Yet another approach to the problem is disclosed in U.S. Pat. No. 4,522,899 where a synthetic plastic, preferably a modified polyolefin having elastomeric characteristics, is initially injection molded in an annular, toroidal O-ring-like configuration about a tapered post. The U.S. Pat. No. 4,522,899 approach relies on shrinkage of the injection molded synthetic polyolefin to provide a seal at the O-ring--post interface. Once the initial O-ring injection molding operation is complete, a second injection molding operation is performed, surrounding the annular, toroidal O-ring configured elastomeric polyolefin with a pot or cylindrical mass of a less elastic material, preferably the same plastic material as the battery case, specifically polyethylene or polypropylene. The pot or cylindrical mass of material is then heat or ultrasonically bonded to the battery case.
While the approach of the U.S. Pat. No. 4,522,899 has superficial attraction, the approach has not proved to be a satisfactory solution to the post sealing problem. The injection molding process by which the annular, toroidal, synthetic plastic O-ring is fabricated about the post does not produce a good seal at the post--synthetic plastic O-ring interface. Indeed, injection molding, being a fabrication process as opposed to a bonding process, has not yielded a satisfactory bond and associated seal between the molded-in-place synthetic plastic O-ring and the metal post. Moreover, the elastomeric materials specified, such as polypropylene, tend to pull away from the battery post as these materials solidify during post-molding cure.
Lead and lead alloys are difficult to bond to other non-lead based metals and to non-metallic materials. Lead oxidizes freely and, hence, always has a small film of lead oxide present, which inhibits bonding. Material selection for post seals is limited because lead corrodes in base environments. The post sealing problem is particularly acute at the positive terminal post in a lead-acid battery because porous lead peroxide forms at that terminal. The sulfuric acid electrolyte easily creeps through the porous lead peroxide. (Surface tension of the acid electrolyte causes the acid to creep across and along all kinds and configurations of surfaces.) The sulfuric acid eventually finds its way through the epoxy polymer resin--lead or lead alloy post interface provided in the newer post seal constructions or creeps around the gaskets and the like used in the older techniques. This results in a leak and/or crevice corrosion, leading to degraded battery performance and battery failure. Leakage about the post with currently used seal techniques may also result from damage to the post seal during battery handling, from gassing in the event the battery is overcharged or from capillary rise of electrolyte along the post.
Batteries are increasingly being used in stand-by applications, to supply power in emergencies for computer systems, telephone switching equipment and the like, in the event of a power dropout of failure of the electric utility. Such stand-by batteries represent a significant capital investment and must have a useful life of many, as opposed to several, years. The required extended life aggravates the problem of providing an adequate, reliable and long lasting post seal because of the longer period over which the corrosive environment in the battery has access to the post seal construction.
Japanese Pat. No. 57-103257 discloses a storage battery cell post sealing structure which includes a hollow river inserted in a gap in the battery cover. A conductive portion of the battery plate extends upwardly around and surrounds the hollow rivet. The conductive portion is surrounded by a porous elastic ring containing an epoxy thermosetting sealing agent consisting of ferrite. A conical head section of the rivet shaft inserted into the hollow river expands the hollow rivet circumferentially outwardly, which presses the hollow rivet against the conductive member and the elastic ring, thereby creating a seal.
United Kingdom Pat. No. 2,133,609 discloses an electric storage cell including a battery post extending upwardly through a hole in the lid of the battery. The lid of the battery has a concave well which contains an elastomeric O-ring extending around the battery post. A terminal strip similarly extends around the battery post on top of the O-ring, with a free end extending outwardly above the top surface of the battery lid. Epoxy resin fills the remainder of the concave well, sealing the battery post with only the free end remaining clear.
United Kingdom Pat. No. 2,084,390 discloses a gas and liquid-tight pole packing. A battery pole extends upwardly through a concave well formed in the battery lid. The well contains three layers of sealing material surrounding the battery pole. The lowermost layer consists of an acid resistant bicomponent resin, the intermediate layer consists of a hot-melt adhesive, and the upper layer consists of an acid resistant bicomponent resin.
U.S. Pat. No. 2,892,006 discloses terminal seals for alkaline electric batteries, wherein a terminal member extends upwardly through a gap in the metallic wall of the battery. Epoxy resin sealing layers circumferentially surround the upwardly extending terminal member and separate the battery casing wall and three sealing washers. One of the sealing washers is located on the inside the battery, while the remaining two sealing washers reside outside the battery, with one washer lying in a well in the top of the battery casing and the other washer lying on top of an adjacent washer in the well.
European patent application No. 113,399 discloses a gas and fluid tight battery having metallic poles, including a battery pole extending upwardly through a hole in a concavely shaped well in the top of the battery. A small water-tight seal surrounds the post at the gap in the battery lid well and an epoxy resin surrounds the remainder of the battery post extending through the well. An upwardly extending flange surrounds the epoxoy resin which in turn is surrounded by three O-rings.
German Pat. No. 3,230,628 discloses a battery post seal wherein a metallic battery post extends upwardly through a gap in the bottom of a well formed in the top of the battery. The exterior of the metallic post is covered with a metallic alloy which contacts an intermediate packing material in the form of O-rings or in the form of a single gasket surrounding the pole and the alloy.
German Pat. No. 1,213,904 discloses a battery post seal, which includes a metallic battery post extending upwardly through a hole in the battery lid. A leakproof dense substance surrounds the battery post along the length of the post extending through the gap. The post has a plurality of flanges which are covered with the dense liquid-proof substance, which in turn engages the corresponding flanges in the top of the battery.
German Pat. No. 1,934,017 discloses a battery having an upwardly extending separator with the top of the separator having an epoxy resin layer thereover. A terminal lies over the epoxy layer, and a portion of the terminal is sealed with a cover and molten packing material.
French Pat. No. 2,112,722 discloses a water-tight seal in batteries which includes a battery terminal extending through a hole in a molded battery lid. The terminal has a groove formed therein which is sized to receive a joint or gasket embedded in the molded lid, thereby creating a seal.